Chapter 18 Thermodynamics and Equilibrium

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Chapter 18 Thermodynamics and Equilibrium Powered By Docstoc
					   Chapter 18
Thermodynamics
 and Equilibrium
Contents and Concepts

1. First Law of Thermodynamics

Spontaneous Processes and Entropy
A spontaneous process is one that occurs by itself. As we
will see, the entropy of the system increases in a
spontaneous process.
2. Entropy and the Second Law of
   Thermodynamics
3. Standard Entropies and the Third Law of
   Thermodynamics

                                                        18 | 2
Free-Energy Concept
The quantity DH – TDS can function as a criterion for the
spontaneity of a reaction at constant temperature, T, and
pressure, P. By defining a quantity called the free energy,
G = H – TS, we find that DG equals the quantity DH – TDS,
so the free energy gives us a thermodynamic criterion of
spontaneity.


4. Free Energy and Spontaneity
5. Interpretation of Free Energy


                                                         18 | 3
Free Energy and Equilibrium Constants
The total free energy of the substances in a reaction
mixture decreases as the reaction proceeds. As we
discuss, the standard free-energy change for a reaction is
related to its equilibrium constant.


6. Relating DG° to the Equilibrium Constant
7. Change of Free Energy with Temperature



                                                         18 | 4
Learning Objectives



1. First Law of Thermodynamics; Enthalpy
    a. Define internal energy, state function, work,
       and first law of thermodynamics.
    b. Explain why the work done by the system
       as a result of expansion or contraction
       during a chemical reaction is -PDV.



                                                  18 | 5
1. First Law of Thermodynamics; Enthalpy
    (cont)
     c. Relate the change of internal energy, DU,
            and heat of reaction, q.
     d. Define enthalpy, H.
     e. Show how heat of reaction at constant
         pressure, qp, equals the change of
         enthalpy, DH.




                                                18 | 6
Spontaneous Processes and Entropy
2. Entropy and the Second Law of
    Thermodynamics
     a. Define spontaneous process.
     b. Define entropy.
     c. Relate entropy to disorder in a molecular
        system (energy dispersal).
     d. State the second law of thermodynamics in
        terms of system plus surroundings.



                                               18 | 7
2. Entropy and the Second Law of
    Thermodynamics (cont)
     e. State the second law of thermodynamics in
        terms of the system only.
     f. Calculate the entropy change for a phase
        transition.
     g. Describe how DH - TDS functions as a
        criterion of a spontaneous reaction.




                                               18 | 8
3. Standard Entropies and the Third Law of
    Thermodynamics
     a. State the third law of thermodynamics.
     b. Define standard entropy (absolute
        entropy).
     c. State the situations in which the entropy
        usually increases.
     d. Predict the sign of the entropy change of a
        reaction.
     e. Express the standard change of entropy of
        a reaction in terms of standard entropies of
        products and reactants.
     f. Calculate DSo for a reaction.
                                                  18 | 9
Free-Energy Concept
4. Free Energy and Spontaneity
     a. Define free energy, G.
     b. Define the standard free-energy change.
     c. Calculate DGo from DHo and DSo.
     d. Define the standard free energy of
        formation, DGo.
     e. Calculate DGo from standard free energies
        of formation.
     f. State the rules for using DGo as a criterion
        for spontaneity
     g. Interpret the sign of DGo.
                                                  18 | 10
5. Interpretation of Free Energy
     a. Relate the free-energy change to
        maximum useful work.
     b. Describe how the free energy changes
        during a chemical reaction.




                                               18 | 11
Free Energy and Equilibrium Constants
6. Relating DGo to the Equilibrium Constant
    a. Define the thermodynamic equilibrium
        constant, K.
    b. Write the expression for a thermodynamic
        equilibrium constant.
    c. Indicate how the free-energy change of a
        reaction and the reaction quotient are
        related.


                                              18 | 12
6. Relating DGo to the Equilibrium Constant
    (cont)
     d. Relate the standard free-energy change to
        the thermodynamic equilibrium constant.
     e. Calculate K from the standard free-energy
        change (molecular equation).
     f. Calculate K from the standard free-energy
        change (net ionic equation).



                                               18 | 13
7. Change of Free Energy with Temperature
    a. Describe how DGo at a given temperature
       (DGoT) is approximately related to DHo and
       DSo at that temperature.
    b. Describe how the spontaneity or
       nonspontaneity of a reaction is related to
       each of the four possible combinations of
       signs of DHo and DSo.
    c. Calculate DGo and K at various
       temperatures.


                                               18 | 14
Thermodynamics is the study of heat and other
forms of energy involved in chemical or physical
processes.




                                                   18 | 15
First Law of Thermodynamics
The first law of thermodynamics is essentially the
law of conservation of energy applied to a
thermodynamic system.

Recall that the internal energy, U, is the sum of
the kinetic and potential energies of the particles
making up the system:
                    DU = Uf – Ui


                                                      18 | 16
Exchanges of energy
between the system and its
surroundings are of two
types: heat, q, and work, w.

Putting this in an equation
gives us the first law of
thermodynamics.

      DU = q + w


                               18 | 17
Sign Convention for q
When heat is evolved by the system, q is negative.
This decreases the internal energy of the system.

When heat is absorbed by the system, q is
positive. This increases the internal energy of the
system.




                                                  18 | 18
Sign Convention for w
Recall from Chapter 6 that w = –PDV.

When the system expands, DV is positive, so w is
negative. The system does work on the
surroundings, which decreases the internal energy
of the system.

When the system contracts, DV is negative, so w is
positive. The surroundings do work on the system,
which increases the internal energy of the system.

                                               18 | 19
Here the system expands and evolves heat from A
to B.




                       Zn2+(aq) + 2Cl-(aq) + H2(g)

       DV is positive, so work is negative.
                                               18 | 20
At constant pressure: qP = DH

The first law of thermodynamics can now be
expressed as follows:
                    DU = DH – PDV




                                             18 | 21
To understand why a chemical reaction goes in a
particular direction, we need to study spontaneous
processes.

A spontaneous process is a physical or chemical
change that occurs by itself. It does not require
any outside force, and it continues until equilibrium
is reached.



                                                  18 | 22
A ball will roll downhill  A ball will not roll uphill
spontaneously. It will     spontaneously. It
eventually reach           requires work.
equilibrium at the bottom.




                                                         18 | 23
The first law of thermodynamics cannot help us to
determine whether a reaction is spontaneous as
written.

We need a new quantity—entropy.

Entropy, S, is a thermodynamic quantity that is a
measure of how dispersed the energy of a system
is among the different possible ways that system
can contain energy.


                                               18 | 24
Examining some spontaneous processes will
clarify this definition.

First, heat energy from a cup of hot coffee
spontaneously flows to its surroundings—the table
top, the air around the cup, or your hand holding
the cup. The entropy of the system (the cup of hot
coffee) and its surroundings has increased.



                                                18 | 25
The rock rolling down the hill is a bit more
complicated. As it rolls down, the rock’s potential
energy is converted to kinetic energy. As it collides
with other rocks on the way down, it transfers
energy to them. The entropy of the system (the
rock) and its surroundings has increased.




                                                  18 | 26
Now consider a gas in a flask connected to an
equal-sized flask that is evacuated. When the
stopcock is open, the gas will flow into the
evacuated flask. The kinetic energy has spread
out, and the entropy of the system has increased.




                                                18 | 27
In each of the preceding examples, energy has
been dispersed (that is, spread out).




                                                18 | 28
Entropy is a state function. It depends on
variables, such as temperature and pressure, that
determine the state of the substance.

Entropy is an extensive property. It depends on the
amount of substance present.




                                                18 | 29
Entropy is measured in units of J/K.

Entropy change is calculated as follows:

                   DS = Sf – Si




                                           18 | 30
The iodine has spread out, so its entropy has
increased.




                                                18 | 31
Second Law of Thermodynamics
The total entropy of a system and its surroundings
always increases for a spontaneous process.

Note: Entropy is a measure of energy dispersal,
not a measure of energy itself.




                                                  18 | 32
For a spontaneous process at a constant
temperature, we can state the second law of
thermodynamics in terms of only the system:

                                    q
             DS = entropy created +
                                    T

For a spontaneous process:
                          q
                     DS >
                          T


                                              18 | 33
A pendulum is put in motion, with all of its molecules
moving in the same direction, as shown in Figures A and B.
As it moves, the pendulum collides with air molecules.
When it comes to rest in Figure C, the pendulum has
dispersed its energy. This is a spontaneous process.

                                                       18 | 34
Now consider the reverse process, which is not
spontaneous. While this process does not violate the first
law of thermodynamics, it does violate the second law
because the dispersed energy becomes more concentrated
as the molecules move together.

                                                       18 | 35
Entropy and Molecular Disorder
Entropy is essentially related to energy dispersal.
The entropy of a molecular system may be
concentrated in a few energy states and later
dispersed among many more energy states. The
energy of such a system increases.




                                                  18 | 36
In the case of the cup of hot coffee, as heat moves
from the hot coffee, molecular motion becomes
more disordered. In becoming more disordered,
the energy is more dispersed.




                                                 18 | 37
Likewise, when the gas moves from one container
into the evacuated container, molecules become
more disordered because they are dispersed over
a larger volume. The energy of the system is
dispersed over a larger volume.




                                             18 | 38
When ice melts, the molecules become more
disordered, again dispersing energy more widely.

When one molecule decomposes to give two, as in
                N2O4(g)  2NO2(g)
more disorder is created because the two
molecules produced can move independently of
each other. Energy is more dispersed.



                                               18 | 39
In each of these cases, molecular disorder
increases, as does entropy.

Note: This understanding of entropy as disorder
applies only to molecular situations in which
increasing disorder increases the dispersion of
energy. It cannot be applied to situations that are
not molecular—such as a desk.



                                                  18 | 40
Entropy Change for a Phase Transition
In a phase transition process that occurs very
close to equilibrium, heat is very slowly absorbed
or evolved. Under these conditions, no significant
new entropy is created.

                            q
                       DS =
                            T

This concept can be applied to melting using DHfus
for q and to vaporization using DHvap for q.


                                                 18 | 41
    Acetone, CH3COCH3, is a volatile liquid

?   solvent; it is used in nail polish, for example.
    The standard enthalpy of formation, DHf°, of
    the liquid at 25°C is –247.6 kJ/mol; the same
    quantity for the vapor is –216.6 kJ/mol.

    What is the entropy change when 1.00 mol
    liquid acetone vaporizes at 25°C?




                                                 18 | 42
        CH3COCH3(l)  CH3COCH3(g)
DHf°    –247.6 kJ/mol –216.6 kJ/mol
n           1 mol          1 mol
nDHf°     –247.6 kJ      –216.6 kJ
                DH° = 31.0 kJ
                      DH
                 ΔS 
                      T
                  31.0  10 3 J
             ΔS 
                    298 K
               DS = 104 J/K

                                      18 | 43
Criterion for a Spontaneous Reaction
The criterion is that the entropy of the system and
its surroundings must increase.
                              q
                        ΔS 
                              T
                            ΔH
                       ΔS 
                             T
                    TΔS  ΔH

         0 ΔH - TΔS or ΔH - TΔS  0


                                                  18 | 44
Third Law of Thermodynamics
A substance that is perfectly crystalline at zero
Kelvin (0 K) has an entropy of zero.




                                                    18 | 45
The standard entropy of a substance—its absolute
entropy, S°—is the entropy value for the standard
state of the species. The standard state is
indicated with the superscript degree sign.

For a pure substance, its standard state is 1 atm
pressure. For a substance in solution, its standard
state is a 1 M solution.




                                                 18 | 46
18 | 47
18 | 48
Standard Entropy of Bromine, Br2, at Various Temperatures




                                                      18 | 49
Entropy Change for a Reaction
Entropy usually increases in three situations:
  1. A reaction in which a molecule is broken into
     two or more smaller molecules.
  2. A reaction in which there is an increase in
     the number of moles of a gas.
  3. A process in which a solid changes to a
     liquid or gas or a liquid changes to a gas.



                                                18 | 50
       The opening of Chapter 6, on thermo-

?      chemistry, describes the endothermic
       reaction of solid barium hydroxide
       octahydrate and solid ammonium nitrate:
Ba(OH)2  8H2O(s) + 2NH4NO3(s) 
              2NH3(g) + 10H2O(l) + Ba(NO3)2(aq)

Predict the sign of DS° for this reaction.

 3 moles of reactants produces 13 moles of
 products. Solid reactants produce gaseous,
 liquid, and aqueous products.
                   DS° is positive.
                                                 18 | 51
To compute DS° where n = moles:

           nS (products)  nS (reactants)
                                 
   ΔS 
      




                                              18 | 52
          When wine is exposed to air in the


?
          presence of certain bacteria, the ethyl
          alcohol is oxidized to acetic acid, giving
          vinegar. Calculate the entropy change
          at 25°C for the following similar
          reaction:

 CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)

The standard entropies, S°, of the substances in
J/(K  mol) at 25°C are CH3CH2OH(l),161; O2(g),
205; CH3COOH(l), 160; H2O(l), 69.9.


                                                  18 | 53
     CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)
S° J/(mol  K) 161     205        160   69.9
n mol          1        1          1     1
nS° J/K       161      205       160    69.9


             DS = 229.9 J/K – 366 J/K

                     DS = –136 J/K


                                           18 | 54
Free Energy and Spontaneity
Physicist J. Willard Gibbs introduced the concept
of free energy, G. Free energy is a
thermodynamic quantity defined as follows:

                   G = H – TS




                                                18 | 55
As a reaction proceeds, G changes:
                  DG = DH – TDS

Standard free energy change:
                DG° = DH° – TDS°




                                     18 | 56
    Using standard enthalpies of formation, DHf°
    and the value of DS° from the previous
?   problem, calculate DG° for the oxidation of
    ethyl alcohol to acetic acid.

CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)




                                              18 | 57
      CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)


DHf° kJ/mol –277.6   0      –487.0       –285.8
n mol          1     1         1            1
nDHf° kJ    –277.6   0      –487.0       –285.8
                –277.6 kJ            –772.8 kJ

                 DH° = –495.2 kJ
                 DS° = –136 J/K
                   T = 298 K

                                                  18 | 58
          DH° = –495.2 kJ
    DS° = –136 J/K = –0.136 kJ/K
             T = 298 K

         DG° = DH° – TDS°
DG° = –495.2 kJ – (298 K)(–0.136 kJ/K)
      DG° = –495.2 kJ + 40.5 kJ

           DG° = –454.7 kJ
     The reaction is spontaneous.


                                         18 | 59
Standard Free Energies of Formation, DGf°
The standard free energy of formation is the free-
energy change that occurs when 1 mol of
substance is formed from its elements in their
standard states at 1 atm and at a specified
temperature, usually 25°C.

The corresponding reaction for the standard free
energy of formation is the same as that for
standard enthalpy of formation, DHf°.


                                                   18 | 60
18 | 61
18 | 62
To find the standard free energy change for a
reaction where n = moles:

          nΔG (products)  nΔG (reactants)
                                   
  ΔG 
     




                                                18 | 63
    Calculate the free-energy change, DG°, for

?   the oxidation of ethyl alcohol to acetic acid
    using standard free energies of formation.

CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)




                                                    18 | 64
        CH3CH2OH(l) + O2(g)  CH3COOH(l) + H2O(l)

DGf°, kJ/mol   –174.8   0        –392.5     –237.2
 n, mol           1     1           1          1
nDGf°, kJ      –174.8   0        –392.5     –237.2

                –174.8 kJ            –629.7 kJ

                DG° = –629.7 – (–174.8)

                  DG° = –454.9 kJ

                                                 18 | 65
DG° as a Criterion for Spontaneity
The spontaneity of a reaction can now be
determined by the sign of DG°.

   DG° < –10 kJ: spontaneous

   DG° > +10 kJ: nonspontaneous

   DG° = very small or zero (< +10 kJ and
         > –10 kJ): at equilibrium

                                            18 | 66
                                  ; DG° = 173.2 kJ




For the reaction as written, DG° = 173.20 kJ.
For 1 mol NO(g), DGf° = 86.60 kJ/mol.
The reaction is nonspontaneous.


                                                     18 | 67
18 | 68
18 | 69
Interpreting Free Energy
Theoretically, spontaneous reactions can be used
to perform useful work. In fact, we use reactions
such as the combustion of gasoline to move a
vehicle.

We can also use spontaneous reactions to provide
the energy needed for a nonspontaneous reaction.

The maximum useful work is
                 wmax = DG

                                               18 | 70
The thermodynamic equilibrium constant is the
equilibrium constant in which the concentrations of
gases are expressed as partial pressures in
atmospheres and the concentrations of solutes in
solutions are expressed in molarities.

If only gases are present, K = Kp.

If only solutes in liquid solution are present,
K = Kc.


                                                  18 | 71
    Write the expression for the thermodynamic


?
    equilibrium constant for these reactions:

    a. N2O4(g)     2NO2(g)
    b. Zn(s) + 2H+(aq)   Zn2+(aq) + H2(g)
                         2
                        PNO2
              a.   K
                        PN2O4


                        [Zn 2 ] PH
              b.   K                 2

                          [H  ] 2


                                             18 | 72
Standard free energy change is related to the
thermodynamic equilibrium constant, K, at
equilibrium:
               DG = DG° + RT ln Q

At equilibrium:
                  DG = 0 and Q = K

                   DG° = –RT ln K



                                                18 | 73
      Calculate the value of the thermodynamic

?     equilibrium constant at 25°C for the reaction
                  N2O4(g)       2NO2(g)
      The standard free energy of formation at
      25°C is 51.30 kJ/mol for NO2 and 97.82
      kJ/mol for N2O4(g).


DG° = 2 mol(51.30 kJ/mol) – 1 mol(97.82 kJ/mol)
          DG° = 102.60 kJ – 97.82 kJ
                DG° = 4.78 kJ

                                                  18 | 74
      DG = –RT ln K
               DG
       ln K 
               RT
                        J
         (4.78  103
                           )
ln K                  mol
                J
       8.315         (298 K)
              mol K

      ln K  1.929

         K  0.145

                               18 | 75
     Sodium carbonate, Na2CO3, can be

?    prepared by heating sodium hydrogen
     carbonate, NaHCO3:

    2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g)

Estimate the temperature at which the reaction
proceeds spontaneously at 1 atm.

See Appendix C for data.



                                            18 | 76
          2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO(g)
DHf°, kJ/mol –947.7 –1130.8 –241.8     –393.5
n, mol          2       1         1         1
nDHf°, kJ –1895.4 –1130.8 –241.8       –393.5

           –1895.4 kJ        –1766.1 kJ
                DH° = 129.3 kJ

Sf°, J/mol  K 102     139       188.7       213.7
n, mol           2       1         1           1
nSf°, J/K      204     139       188.7       213.7

              204 J/K            541.4 J/K
                  DS° = 337.4 J/K
                                                     18 | 77
        ΔH 
    T 
        ΔS

   129.3  103 J
T
            J
     337.4
            K

   T  383 K

   T 110C

                   18 | 78

				
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